1. Field of the Invention
The present invention relates to erecting equal-magnification lens array plates used in image reading devices and image forming devices and to image sensor units and image reading devices using the erecting equal-magnification lens array plate.
2. Description of the Related Art
Some image reading devices such as scanners and image forming devices such as LED printers are known to use erecting equal-magnification optics. Erecting equal-magnification optics are capable of reducing the size of devices better than reduction optics. In the case of image reading devices, an erecting equal-magnification optical system comprises a line light source, an erecting equal-magnification lens array, and a line image sensor. In the case of image forming devices, an erecting equal-magnification optical system comprises a line light source, an erecting equal-magnification lens array, and a photosensitive drum.
A rod lens array capable of forming an erect equal-magnification image is used as an erecting equal-magnification lens array in an erecting equal-magnification optical system. Normally, a rod lens array comprises an array of rod lenses in the longitudinal direction (main scanning direction of the image reading device) of the lens array. By increasing the number of columns of rod lenses, the proportion of light transmitted is improved and unevenness in the amount of light transmitted is reduced. Due to price concerns, it is common to use one or two columns of rod lenses in a rod lens array.
Meanwhile, an erecting equal-magnification lens array plate could be formed as a stack of a plurality of transparent lens array plates built such that the optical axes of individual convex lenses are aligned, where each transparent lens array plate includes a systematic arrangement of micro-convex lenses on one or both surfaces of the plate. Since an erecting equal-magnification lens array plate such as this can be formed by, for example, injection molding, erecting equal-magnification lens arrays in a plurality of columns can be manufactured at a relatively low cost.
An erecting equal-magnification lens array plate lacks a wall for beam separation between adjacent lenses. Therefore, there is a problem of stray light wherein a light beam diagonally incident on an erecting equal-magnification lens array plate travels diagonally inside the plate and enters an adjacent convex lens, creating a ghost image as it leaves the plate.
Patent document No. 1 discloses a technology to address stray light whereby a light shielding wall is provided on the surface of an erecting equal-magnification lens array plate and a partition having a slit opening is provided around the erecting equal-magnification lens array plate. Patent document No. 2 discloses an imaging optical system provided with a light shielding means on an intermediate imaging surface of an erecting equal-magnification lens array plate.    [patent document No. 1] JP 2005-37891    [patent document No. 2] JP 2005-122041
In the case of the imaging optical system disclosed in patent document No. 1, however, it is difficult to reduce the size and weight of the optical system due to the partition having a slit opening and provided around the erecting equal-magnification lens array plate.
In the case of an imaging optical system disclosed in patent document No. 2, stray light in the sub-scanning direction (lateral direction of the erecting equal-magnification lens array plate) can be eliminated by the light shielding means. Our study revealed, however, that it is difficult to eliminate stray light in the main scanning direction.